ES cells are embryonic stem cells that were discovered in mouse embryos in 1981 and in human embryos in 1998. ES cells having the ability to develop into a variety of types of cells (i.e., pluripotent cells) except for the cells that constitute placenta have been primarily studied for the construction of tissues or organs therefrom. Because of the use of the fertilized eggs that would develop into new lives if they were allowed to grow smoothly, ES cells raise serious ethical questions. Another serious issue is the problem of rejection. When differentiated cells or organs prepared from ES cells are transplanted into a patient, the immune system of the patient may recognize such cells or organs as foreign substances and attack them.
In order to overcome the problems of ES cells, Professor Shinya Yamanaka et al. at Kyoto University developed cells capable of developing into various types of cells from dermal cells that are not generally differentiated into cells exerting other functions, and they designated these cells “iPS cells.” They demonstrated that introduction of four factors referred to as “Yamanaka factors;” i.e., Oct 3/4, Sox2, Klf4, and c-Myc, into mouse or human dermal cells with the use of a retrovirus vector would lead to reprogramming of cells and production of pluripotent cells, as is the case with ES cells (Non-Patent Document 1: Takahashi and Yamanaka, Cell 126, 663-676, 2006; and Non-Patent Document 2: Takahashi et al., Cell 131, 861-872, 2007). Since the cells used in such case are derived from somatic cells, such as differentiated dermal cells, of the patient him/herself, the immune system would recognize an organ prepared from the cells differentiated from the iPS cells as an autonomous organ upon transplantation thereof, and the transplant would accordingly not be rejected. As a result of the discovery of iPS cells, the issue of ethical concern regarding ES cells was overcome.
As described above, iPS cells have drawn attention worldwide as a powerful tool for regenerative medicine, although the technical problem of canceration of cells remains problematic. A cause of canceration is related to the introduction of the c-Myc gene into cells; however, iPS cells have been produced from the other 3 factors than the c-Myc gene in recent years. When introducing a gene into a cell, iPS cells were prepared with the use of the adenovirus or plasmid vector instead of the retrovirus vector. This allowed for the production of iPS cells with advanced safety and usefulness. However, this technique involves the artificial and forced expression of several genes in cells that had completed differentiation, and the risk that such cells would experience canceration in the future cannot be denied.
Meanwhile, Patent Document 1 describes a method of using Mycobacterium leprae or a component thereof so as to produce reprogrammed embryonic stem (ES)-like cells. Specifically, Patent Document 1 describes a method for producing reprogrammed ES-like cells comprising bringing Mycobacterium leprae or a component thereof into contact with a differentiated cell derived from an adult, and it also describes cells produced by such method. However, Mycobacterium leprae is a lepra bacillus, and application thereof to regenerative medicine remains problematic in terms of safety.